What get inverted, or turned over in a thermal inversion?

In normal atmosphere, temperature decreases with increasing altitude ...
it gets colder as you go up.

During relatively unusual conditions, a mass of warmer air sits on top of
the cooler air below.  The temperature increases with increasing altitude ...
it gets warmer as you go up.

That's the unusual, abnormal situation, called a "temperature inversion".

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).